Thermo King KD-II SR, Manual

Page: 65 / 182

Electrical Maintenance (Rev. 04/03)
51
KD-II SR Units Equipped with a
Three-Phase Electric Motor and a Phase
Converter That Operates on Single-Phase
Power
NOTE: The drawings used in this discussion are typical to
provide a general overview of operation. For exact draw-
ing references, refer to 5D45076 and 5D48889 diagrams
in the back of this manual.
The KD-II SR units that are ordered with the single-phase
220 volt electric standby option are equipped with a three-
phase electric motor and a phase conversion system. This
electric motor is the same 5/4.2 hp (3.7/3.1 kW) motor used
in KD-II units with a three-phase electric standby option.
The phase conversion system enables the three-phase elec-
tric motor to operate using power from a single-phase
power source.
NOTE: The suction pressure regulator must be set at a
maximum of 19 psig (131 kPa) on these single-phase
units. A higher setting will overload the electric motor. To
check the suction pressure regulator setting, run the unit
on defrost and observe the suction pressure.
The overload relay is set at 18 amps. A higher setting may
not provide adequate protection.
Voltage taps (230 volt or 208 volt) for the transformer are
located on the test strip in the control panel. To make sure
the unit operates properly, wire L1A should be connected to
the voltage tap that matches the voltage of the power source.
Wire L1A is normally connected to the 208 volt tap.
Conversion System
The phase conversion system itself consists of a start system
and a run system. The start system is energized by the high
current draw that is present while the motor is starting under
a load. Once the motor is running at normal speed and the
current draw has dropped, the start system is de-energized
and the run system is solely responsible for the phase shift.
The run system is always connected to the motor circuit, but
it does not affect the circuit while the start system is ener-
gized because the start capacitors have much larger capaci-
tance and dominate the run capacitors.
Start System
The components of the start system are:
• a current relay
• a motor start relay
• a motor start contactor
• a filter capacitor
• two start capacitors (400 to 480
µ
F each)
• two start bleed down resistors (180 k ohms each)
Single-phase power is available at L1 and L2. L1 and L2
pass through the motor contactor and become L1A and
L2A. L1A passes through the overload relay to become T1.
L2A passes through the overload relay to become L2B. L2B
passes through the overload relay to become T2.
The three-phase motor has two separate sets of windings
with three windings in each set. Branches of T1 are con-
nected directly to the first winding in each set. A branch of
T2 is connected directly to the second winding in one set.
Another branch of T2 passes through the control coil of the
current relay and becomes T2A, which is connected to the
second winding in the other set.